It is known to use in several products so-termed “brighteners”. The brighteners absorb a portion of the light (e.g. UV light) that impinges on them and converts the absorbed light towards light of another color. This additional light of another color results in the fact that the human naked eye experiences the color of the object as a more appealing color, as a more crispy color. For example, brighteners are added to white paper such that the paper appears to be whiter to the human naked eye. The light that is reflected by the white paper in combination with the light generated by the brighteners is often termed “crispy white light”. If this white paper with brighteners is laid beside a 100% reflecting white object (without brighteners), the human naked eye experiences the white paper as more white and experiences the white of the 100% reflecting white object as an object that is slightly grey or slightly yellow/orange. The effect of the brighteners can be well seen under natural daylight, and, for example, high pressure incandescent lamps. However, most LED based light sources do not reveal the effect of the brighteners and studies have shown that in particular Ultraviolet (more in particular, UVA light) or violet light excite the brighteners. LED based light sources do, in general, not emit much light in these UVA or violet spectral ranges.
Published patent application WO2013/150470, which is included by reference, provides a solution for the problem that most LED based light sources emit light that does not lead to the excitation of light by the brighteners. According to WO2013/150470, in a light source which comprises a phosphor converted light emitter (for emitting, for example, white light), an additional light emitter is provided which emits violet light in the spectral range from 400 to 440 nanometer such that, when the emitted light impinges on an object with brighteners, the brighteners absorb the violet light and emit the light of another color.
With the solution of the cited patent application, one still has to add brighteners to the product that has to look more appealing and/or more crispy when being illuminated by the light source of the patent application.